The head of a Metropolitan police station where a man died has been recorded on tape promising that CCTV cameras were working and tape from them had been seized – weeks before investigators told the man's family that the footage was missing.

Sean Rigg, a 40-year-old musician, was arrested in the street by four police officers, placed in a police van and driven a short distance to Brixton police station, where he was placed in a metal cage in the yard. About 20 minutes later he stopped breathing and after being taken to hospital was officially pronounced dead.

Rigg had schizophrenia, an illness his family say he overcame in recent years to live a normal life. He was otherwise physically fit and healthy around the time he died. Two postmortem examinations were unable to find a cause of death.

Rigg's family believe officers who restrained him during the arrest, or those present when he was placed in the station cage, may have contributed to his death. They have held a weekly vigil outside the station and were to mark the anniversary of his death today with a memorial event.

There were no cameras in the police van that took Rigg to the police station and the Independent Police Complaints Commission (IPCC), which is expected to complete its investigation next month, initially told Rigg's family that only CCTV footage seized from inside the station showed the cage where he died – and the cameras involved had limited views.

Convinced there were more outdoor cameras nearby, Rigg's family demanded an audit of security cameras at the station. IPCC investigators then conceded there were more cameras overlooking the cage. But two weeks later, they said they had tried to obtain the tapes and found the recorders had not been working for three months.

Rigg's family suspect a cover-up. The IPCC's claim about CCTV contradicts repeated assurances given to the family by a senior police officer two days after Rigg died. Suzanne Wallace, a chief inspector who was in charge of the station, was caught on tape saying CCTV was working and recordings had been seized.

"I know that's been seized because on the night the officers from the DPS [Department of Professional Standards] were asking about the CCTV," she said. "And that was one of my questions: is the CCTV working, is it running, is there a tape in there? You know – very basic, but it's really, really important. And that was all to the affirmative.

"So the CCTV was running and they were certainly, they certainly seized it on the night cos they were looking to do that safely so they wouldn't mess the system up or delete anything that was on the tape. So that has been done, so it was running, so there is CCTV and the IPCC will have that if they haven't got that now."

Specifically, she said "the back of the cage and the whole yard is covered with CCTV" and later in the conversation added: "My worst nightmare is the CCTV wasn't working, because I can imagine then you'd think: Oh my God [inaudible] cover-up, you've wiped the tape, the officers [inaudible]. Of course you'd think that, I would think that."

The Met declined to comment as the case is part of an ongoing IPCC investigation.

Ignored 999 calls

For Rigg's family, the missing CCTV footage raises serious questions about the actions of police on 21 August last year. They want to know why he was left for an hour in an outdoor cage, which functions as a station holding area, rather than taken into the custody suite.

One theory held by the family is that officers knew the cage was in a CCTV "blind spot" and left Rigg there so there would be less evidence of his deteriorating condition upon arrival at the station.

Another is that the tapes were destroyed during the 27 days it took the IPCC to attempt to seize footage from outdoor cameras.

Rigg's sister Marcia, 45, said: "It is my opinion that this is a deliberate cover-up by the police, and the IPCC [by failing to rigorously investigate] are allowing that cover-up. It's all part of collusion and to me the IPCC are certainly not independent."

Records show some cameras at Brixton police station were reported as faulty. However an annual maintenance check of all CCTV completed on 12 August, nine days before Rigg died, found no problems with the cameras that the family believe should have recorded Rigg's last moments alive.

Rigg is known to have missed a dose of medication prior to his death and appeared to have suffered a psychotic episode at a Brixton hostel for people with mental health issues where he lived.

Concerned about his mental state, staff at the hospital made several 999 calls to police over a period of two hours. Police failed to send out a unit and at 7pm Rigg left the hostel, alone and wearing just trousers and shoes.

Four police officers, two of them trainees, went looking for him, apparently after a call from a member of the public who had spotted Rigg behaving strangely on the street.

The officers found Rigg at about 7.40pm and chased him through a nearby estate where he was arrested on suspicion of a public order offence and assaulting an officer during the arrest. Rigg's family believe he sustained several injuries either before or while being placed the police van, where there were no cameras.

The van entered the police yard at 7.53pm and Rigg was left inside for about 10 minutes before officers escorted him to the cage. CCTV inside the station's custody area recorded obscured footage of Rigg in the cage. His family, who have watched the images, say they show him collapse repeatedly and lose consciousness.

"It was really upsetting to see the footage that we did see," said his sister, Samantha Rigg-David, 43. "It just didn't seem that Sean was afforded the care that any human being deserved. He was placed on the cold floor and that's precisely where Sean died. He didn't move from that area. He wasn't carried inside the police station, into the warm. He appears to be going in and out of consciousness and then he's lifeless. That's where Sean died."

The family say that in the footage they were shown an officer nudges Rigg with a foot, as if to check he is still conscious and reactive.

They allege the custody footage records conversations between officers including one saying Rigg was "feigning" fitting and unconsciousness. Another officer, they claim, can be heard clearly saying: "If he dies in here we're all in the shit."

Scratch on his face

"The IPCC has repeatedly refused to contemplate that the officers involved could have acted negligently or with malice," said the Rigg family's lawyer, Anna Mazzola, from Hickman and Rose. "It has singularly failed to treat this as a suspicious death despite the fact that there is no explanation for how a physically healthy man collapsed and died in police custody within minutes of coming into contact with police officers."

The family said they were denied permission to see Rigg's body for two days and told that he had an injured foot, grazing to his elbows and knees and a scratch on his face.

When the family arrived at the mortuary they persuaded staff to unbolt a glass screen that stopped them seeing his whole face. They claim they discovered three round wounds to the side of his face – on his eyebrow, cheek and temple – although a pathologist judged these could not have killed him..

"If we had not gone in there we would perhaps still – a year later – not have known anything about those wounds," said Rigg's brother Wayne, 39. "There seems to be a strong bias towards police."

IPCC investigators waited eight months to interview police officers who came into contact with Rigg the night he died, and nine months before speaking to the 999 call handlers. Investigators failed to seal off the area where Rigg was arrested prior to being taken to the station.

The IPCC replaced the senior investigator in the case, Chris Patridge, after acknowledging the family's lack of confidence in him. The commission has apologised for releasing a misleading press statement about Rigg's death. But in meetings with the family, investigators have defended their actions and said they did not immediately interview all officers because there was nothing to suggest wrongdoing.

The scene of Rigg's arrest was not forensically examined because it was likely to have been contaminated, the IPCC told his family.

In a statement, the IPCC said: "We understand the concerns of Mr Rigg's family regarding the CCTV at Brixton police station and have, therefore, as part of the wider investigation produced a separate report into this matter."

The IPCC said the report had been given to the family but could not be made public until the inquest. "Having taken into consideration the lack of confidence which the family of Sean Rigg has expressed in the IPCC, Commissioner Amerdeep Somal took a number of steps to try to address the family's concerns. One such step was changing the lead investigator."

In the family's view, the IPCC has yet to give an adequate explanation about the missing CCTV. "That was the way we were going to find out what was going to happen that night," said Wayne Rigg. "We were told that the cameras were working. We went and saw the positions of these cameras. Then to be told the cameras weren't working – we were devastated."